Tests/LibWeb: Split input/grid/gap.html into smaller tests

The file gap.html, which previously had multiple grid tests, has now
been divided into smaller files, each containing only one grid test.
It is going to make it easier to identify what inputs have been
affected by changes in layout code.

Tests/LibWeb/Layout/input/grid/grid-gap-1.html

@@ -0,0 +1,24 @@
+<style>
+.grid-container {
+    display: grid;
+    background-color: lightsalmon;
+    grid-template-columns: auto auto;
+    gap: 50px 100px;
+}
+
+.one {
+    background-color: lightblue;
+}
+
+.two {
+    background-color: yellowgreen;
+}
+
+.three {
+    background-color: palevioletred;
+}
+
+.four {
+    background-color: lightseagreen;
+}
+</style><div class="grid-container"><div class="one">1</div><div class="two">2</div><div class="three">3</div><div class="four">4</div></div>